High SBS threshold NZDS optical fiber

ABSTRACT

A non-zero dispersion shifted optical waveguide fiber having a high threshold for stimulated Brillouin scattering is disclosed.

This application claims the benefit of priority under 35 U.S.C. § 119(e)of U.S. Provisional Application Ser. No. 60/525,545 filed on Nov. 26,2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to high SBS threshold optical fibers. Morepreferably, the present invention relates to high SBS threshold non-zerodispersion shifted optical fibers, or NZDS fibers, or NZ-DSF's.

2. Technical Background

Stimulated Brillouin Scattering (SBS) is a dominant nonlinear penalty inmany optical transmission systems. In many systems, the launching oflarge power to optical fiber while maintaining high signal to noiseratio (SNR) is desirable. However, as the launch power or signal powerof an incident signal launched into an optical fiber increases, thelaunch power may exceed a certain threshold power and part of the signalpower gets reflected due to SBS as a reflected signal. An undesirablylarge amount of the signal power can thus be reflected back toward thetransmitter due to SBS. In addition, the scattering process increasesthe noise level at the signal wavelength. The combination of decrease insignal power and increase in the noise both lower the SNR and lead toperformance degradation.

At finite temperatures, thermal excitations occur in glasses similar tophonons in crystals, and the interaction of these vibrational modes withlow intensity signal light produces spontaneous Brillouin scattering. Anintense optical field generates pressure or sound waves throughelectrostriction due to the beating of intense incident and spontaneousreflected light giving rise to pressure or acoustic waves. The change inpressure causes material density to change, thereby resulting inrefractive index fluctuations. The net result is that an intenseelectrical field component of the optical wave generates pressure orsound waves which cause density fluctuations. The acoustic wave changesthe refractive index and enhances the reflected light amplitude throughBragg diffraction. Above the SBS threshold of an optical fiber, thenumber of stimulated photons is very high, resulting in a strongreflected field which limits the optical power that is transmitted andwhich reduces the SNR.

SUMMARY OF THE INVENTION

Disclosed herein is non-dispersion shifted optical fiber (NZDSF fiber)having a high SBS threshold. The optical fiber guides at least oneoptical mode and a plurality of acoustical modes, including an L₀₁acoustical mode and an L₀₂ acoustical mode.

The optical fiber comprises a core having a refractive index profile anda centerline and a cladding layer surrounding and directly adjacent thecore. The core comprises a plurality of segments, preferably threesegments that include a central segment, a moat segment, and a ringsegment. Preferably, the core comprises a maximum relative refractiveindex greater than 0.75%.

The refractive index of the core is selected to provide an opticaleffective area at 1550 nm less than 80 μm²; a dispersion at 1550 nmhaving an absolute magnitude less than 7.5 ps/nm-km; and an absolute SBSthreshold (in dBm) greater than about 9.5+10 log[(1−e^(−(0.19)(50)/4.343))/(1−e^(−(α)(L)/4.343))] dBm for a given fiberlength, L in km, and a given attenuation, a in dB/km, of the opticalfiber, as determined using a continuous wave light source, preferablyhaving a spectral width of less than about 150 kHz, wherein the log isunderstood to be a base 10 logarithm. Preferably, the attenuation at1550 nm is less than 0.23 dB/km, more preferably less than 0.21 dB/km.Embodiments are disclosed herein having either positive or negativedispersion at 1550 nm. Embodiments having an absolute SBS thresholdgreater than about 10+10 log[(1−e^(−(0.19)(50)/4.343))/(1−e^(−(α)(L)/4.343))] dBm, as well asembodiments greater than about 10.5+10 log[(1−e^(−(0.19)(50)/4.343))/(1−e^(−(α)(L)/4.343))] dBm, are alsodisclosed herein. Preferably, the acousto-optic effective area,AOEA_(L01), of the L₀₁ acoustical mode is not less than 150 μm² at theBrillouin frequency of the optical fiber. Preferably, the acousto-opticeffective area, AOEA_(L02), of the L₀₂ acoustical mode is not less than150 μm² at the Brillouin frequency of the optical fiber. Morepreferably, both AOEA_(L01) and AOEA_(L02) are not less than 150 μm² atthe Brillouin frequency of the optical fiber.

In preferred embodiments, the optical effective area is between 50 and80 μm². In other preferred embodiments, the optical effective area isbetween 60 and 80 μm².

In one set of preferred embodiments, optical fiber is disclosed having acore having a refractive index profile and a centerline, and having acladding layer surrounding and directly adjacent the core, wherein therefractive index of the core is selected to provide an optical effectivearea at 1550 nm less than 80 μm², a negative dispersion at 1550 nm, andan absolute SBS threshold greater than 9.5 dBm for fiber lengths greaterthan or equal to about 50 km.

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows refractive index profiles corresponding to a first set ofpreferred embodiments of an optical waveguide fiber as disclosed herein.

FIG. 2 shows refractive index profiles corresponding to a second set ofpreferred embodiments of an optical waveguide fiber as disclosed herein.

FIG. 3 shows refractive index profiles corresponding to a third set ofpreferred embodiments of an optical waveguide fiber as disclosed herein.

FIG. 4 is a schematic cross-sectional view of a preferred embodiment ofan optical waveguide fiber as disclosed herein.

FIG. 5 is a schematic view of a fiber optic communication systememploying an optical fiber as disclosed herein.

FIG. 6 is a schematic of a representative measurement system formeasuring SBS threshold.

FIG. 7 is a plot of backscattered power versus input power, and itsfirst and second derivatives for a representative optical fiber SNSthreshold measurement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Additional features and advantages of the invention will be set forth inthe detailed description which follows and will be apparent to thoseskilled in the art from the description or recognized by practicing theinvention as described in the following description together with theclaims and appended drawings.

The “refractive index profile” is the relationship between refractiveindex or relative refractive index and waveguide fiber radius.

The “relative refractive index percent” is defined as Δ%=100×(n_(i)²−n_(c) ²)/2n_(i) ², where n_(i) is the maximum refractive index inregion i, unless otherwise specified, and n_(c) is the averagerefractive index of the cladding region. As used herein, the relativerefractive index is represented by Δ and its values are given in unitsof “%”, unless otherwise specified. In cases where the refractive indexof a region is less than the average refractive index of the claddingregion, the relative index percent is negative and is referred to ashaving a depressed region or depressed index, and is calculated at thepoint at which the relative index is most negative unless otherwisespecified. In cases where the refractive index of a region is greaterthan the average refractive index of the cladding region, the relativeindex percent is positive and the region can be said to be raised or tohave a positive index. An “updopant” is herein considered to be a dopantwhich has a propensity to raise the refractive index relative to pureundoped SiO₂. A “downdopant” is herein considered to be a dopant whichhas a propensity to lower the refractive index relative to pure undopedSiO₂. An updopant may be present in a region of an optical fiber havinga negative relative refractive index when accompanied by one or moreother dopants which are not updopants. Likewise, one or more otherdopants which are not updopants may be present in a region of an opticalfiber having a positive relative refractive index. A downdopant may bepresent in a region of an optical fiber having a positive relativerefractive index when accompanied by one or more other dopants which arenot downdopants. Likewise, one or more other dopants which are notdowndopants may be present in a region of an optical fiber having anegative relative refractive index.

“Chromatic dispersion”, herein referred to as “dispersion” unlessotherwise noted, of a waveguide fiber is the sum of the materialdispersion, the waveguide dispersion, and the inter-modal dispersion. Inthe case of single mode waveguide fibers the inter-modal dispersion iszero. Zero dispersion wavelength is a wavelength at which the dispersionhas a value of zero. Dispersion slope is the rate of change ofdispersion with respect to wavelength.

Effective area is defined as [G. P. Agrawal, Nonlinear Fiber Optics, 3dedition, Academic Press, 2001, p. 44]

${A_{eff} = \frac{2{\pi\left( {\int{\int{f_{o}^{2}r{\mathbb{d}r}}}} \right)}^{2}}{\int{\int{f_{o}^{4}r{\mathbb{d}r}}}}},$

-   -   where the integration limits are 0 to ∞ for the radial direction        r, and f_(o) is the optical field associated with light        propagated in the waveguide. As used herein, “effective area” or        “A_(eff)” refers to optical effective area at a wavelength of        1550 nm unless otherwise noted.

The term “α-profile” refers to a relative refractive index profile,expressed in terms of Δ(r) which is in units of “%”, where r is radius,which follows from the equation,Δ(r)=Δ(r _(o))(1−[|r−r _(o)|/(r _(l) −r _(o))]^(α)),where r_(o) is the point at which Δ(r) is maximum, r_(l) is the point atwhich Δ(r)% is zero, and r is in the range r_(i)≦r≦r_(f), where Δ isdefined above, r_(i) is the initial point of the α-profile, r_(f) is thefinal point of the α-profile, and α is an exponent which is a realnumber.

The mode field diameter (MFD) is measured using the Peterman II methodwherein, 2w=MFD, and w²=(2∫f_(o) ² r dr/∫[df_(o)/dr]² r dr), theintegral limits being 0 to ∞.

The bend resistance of a waveguide fiber can be gauged by inducedattenuation under prescribed test conditions.

One type of bend test is the lateral load microbend test. In thisso-called “lateral load” test, a prescribed length of waveguide fiber isplaced between two flat plates. A #70 wire mesh is attached to one ofthe plates. A known length of waveguide fiber is sandwiched between theplates and a reference attenuation is measured while the plates arepressed together with a force of 30 newtons. A 70 newton force is thenapplied tot he plates and the increase in attenuation in dB/m ismeasured. The increase in attenuation is the lateral load attenuation ofthe waveguide.

The “pin array” bend test is used to compare relative resistance ofwaveguide fiber to bending. To perform this test, attenuation loss ismeasured for a waveguide fiber with essentially no induced bending loss.The waveguide fiber is then woven about the pin array and attenuationagain measured. The loss induced by bending is the difference betweenthe two measured attenuations. The pin array is a set of ten cylindricalpins arranged in a single row and held in a fixed vertical position on aflat surface. The pin spacing is 5 mm, center to center. The pindiameter is 0.67 mm. During testing, sufficient tension is applied tomake the waveguide fiber conform to a portion of the pin surface.

The theoretical fiber cutoff wavelength, or “theoretical fiber cutoff”,or “theoretical cutoff”, for a given mode, is the wavelength above whichguided light cannot propagate in that mode. A mathematical definitioncan be found in Single Mode Fiber Optics, Jeunhomme, pp. 39–44, MarcelDekker, New York, 1990 wherein the theoretical fiber cutoff is describedas the wavelength at which the mode propagation constant becomes equalto the plane wave propagation constant in the outer cladding. Thistheoretical wavelength is appropriate for an infinitely long, perfectlystraight fiber that has no diameter variations.

The effective fiber cutoff is lower than the theoretical cutoff due tolosses that are induced by bending and/or mechanical pressure. In thiscontext, the cutoff refers to the higher of the LP11 and LP02 modes.LP11 and LP02 are generally not distinguished in measurements, but bothare evident as steps in the spectral measurement, i.e. no power isobserved in the mode at wavelengths longer than the measured cutoff. Theactual fiber cutoff can be measured by the standard 2 m fiber cutofftest, FOTP-80 (EIA-TIA-455-80), to yield the “fiber cutoff wavelength”,also known as the “2m fiber cutoff” or “measured cutoff”. The FOTP-80standard test is performed to either strip out the higher order modesusing a controlled amount of bending, or to normalize the spectralresponse of the fiber to that of a multimode fiber.

The cabled cutoff wavelength, or “cabled cutoff” is even lower than themeasured fiber cutoff due to higher levels of bending and mechanicalpressure in the cable environment. The actual cabled condition can beapproximated by the cabled cutoff test described in the EIA-445 FiberOptic Test Procedures, which are part of the EIA-TIA Fiber OpticsStandards, that is, the Electronics Industry Alliance—TelecommunicationsIndustry Association Fiber Optics Standards, more commonly known asFOTP's. Cabled cutoff measurement is described in EIA-455-170 CableCutoff Wavelength of Single-mode Fiber by Transmitted Power, or“FOTP-170”.

Unless otherwise noted herein, optical properties (such as dispersion,dispersion slope, etc.) are reported for the LP01 mode.

Kappa is the ratio of dispersion divided by dispersion slope at aparticular wavelength. Unless otherwise noted herein, kappa is reportedat a wavelength of 1550 nm.

A waveguide fiber telecommunications link, or simply a link, is made upof a transmitter of light signals, a receiver of light signals, and alength of waveguide fiber or fibers having respective ends opticallycoupled to the transmitter and receiver to propagate light signalstherebetween. The length of waveguide fiber can be made up of aplurality of shorter lengths that are spliced or connected together inend to end series arrangement. A link can include additional opticalcomponents such as optical amplifiers, optical attenuators, opticalisolators, optical switches, optical filters, or multiplexing ordemultiplexing devices. One may denote a group of inter-connected linksas a telecommunications system.

A span of optical fiber as used herein includes a length of opticalfiber, or a plurality of optical fibers fused together serially,extending between optical devices, for example between two opticalamplifiers, or between a multiplexing device and an optical amplifier. Aspan may comprise one or more sections of optical fiber as disclosedherein, and may further comprise one or more sections of other opticalfiber, for example as selected to achieve a desired system performanceor parameter such as residual dispersion at the end of a span.

Various wavelength bands, or operating wavelength ranges, or wavelengthwindows, can be defined as follows: “1310 nm band” is 1260 to 1360 nm;“E-band” is 1360 to 1460 nm; “S-band” is 1460 to 1530 nm; “C-band” is1530 to 1565 nm; “L-band” is 1565 to 1625 nm; and “U-band” is 1625 to1675 nm.

When an optical wave propagates in an optical waveguide in whichacoustic modes are present, the frequency of the scattered light isdetermined by phase and wave-vector matching conditions:

-   -   where E₁ and E₂ are electrical fields of the incident and        reflected optical waves, respectively, ω₁ and ω₂ are the        respective frequencies, κ₁ and κ₂ are the respective optic wave        vectors, ρ is material density, Ω is acoustic frequency, and q        is acoustic wave vector.

The phase matching conditions are:ω₁=ω₂+Ωq=κ ₁−κ₂|q|=κ ₁+κ₂≅2κ₁

The Brillouin frequency is:ω=|q|vω=2nvω ₁ /cThe Brillouin frequency for bulk silica is around 11 GHz and ν isvelocity of sound.

The optical fields that contribute to the Brillouin scattering are:incident field: E ₁(z,t)=K _(o) f _(o)(r,θ)A ₁(z,t)exp[i(κ₁ z−ω ₁t)]+c.c.reflected field: E ₂(z,t)=K _(o) f _(o)(r,θ)A ₂(z,t)exp[i(κ₂ z−ω ₂t)]+c.c.

-   -   where f_(o)(r,θ) is the amplitude of the optical field at radius        r and azimuthal angle θ, K_(o)=[∫∫f_(o) ²(r,θ)rdrdθ]^(−1/2) is        the normalization factor, and “c.c.” denotes the complex        conjugate of the first term.

The material density {tilde under (ρ)} obeys the acoustic wave equation[R. W. Boyd, Nonlinear Optics, 2nd edition, Academic Press 2003, p.418]:

${\frac{\partial^{2}\overset{\sim}{\rho}}{\partial t^{2}} - {\Gamma^{\prime}{\nabla^{2}\frac{\partial\overset{\sim}{\rho}}{\partial t}}} - {{v^{2}(r)}{\nabla^{2}\overset{\sim}{\rho}}}} = {- \frac{{\gamma_{e}\nabla^{2}} < E^{2} >}{8\pi}}$where ∇² is the Laplacian operator, Γ′ is the damping parameter, γ_(e)is the electrostrictive constant, ν is the velocity of sound, andE=E₁+E₂.

The material density change can be represented as:

${\overset{\sim}{\rho}\left( {r,\theta,t,z} \right)} = {\rho_{0} + {\sum\limits_{n}^{\;}\;{{a_{n}\left( {z,t} \right)}{f_{a}^{(n)}\left( {r,\theta} \right)}{\exp\left\lbrack {{\mathbb{i}}\left( {{q_{n}z} - {\Omega_{n}t}} \right)} \right\rbrack}}} + {c.c.}}$where ρ₀ is the material density of silica. f_(a) ^((n))(r,θ) is thetransverse profile of the acoustic mode, which is the nth solution ofthe equation:

${{{\nabla_{\bot}^{2}{- \left( {\frac{\Omega^{2}}{v^{2}(r)} - q^{2}} \right)}}{\rho\left( {r,\theta} \right)}} = 0},$where q_(n) and Ω_(n) are the wave number and the acoustic frequency ofthe nth acoustic mode, respectively, and the coefficient a_(n)(z,t)describes the spatial and temporal evolution of the nth acoustic mode.

The dependence of sound velocity on the transverse coordinate in thefiber ν(r) is determined by the corresponding index profile Δ(r). Themodal equation can be numerically solved by including the radialvariation of the material density and longitudinal sound velocity acrossthe refractive index profile:ν(r)=5944[1−0.12*Δ(r)][m/s]where numerical parameters in above equation were initially taken fromN. Lagakos, J. A. Bucaro, and R. Hughes, Applied Optics, vol. 19, pp.3668–3670 (1980) and adjusted to match the peaks in the measuredBrillouin gain spectrum of LEAF optical fiber manufactured by CorningIncorporated [C.C. Lee and S. Chi, “Repeaterless Transmission of80-Channel AM-SCM Signals over 100 km Large effective areadispersion-shifted fiber,” IEEE Photonics Technology Lett. vol. 12, pp.341–343 (February 2000)].

The above expansion can be substituted in the acoustic wave equation andthe orthogonality condition used to obtain:

${a_{n} \sim \frac{\int{\int{f_{o}^{2}f_{a}^{(n)}r{\mathbb{d}r}{\mathbb{d}\theta}}}}{\int{\int{f_{o}^{2}r{\mathbb{d}r}\;\theta{\int{\int{\left( f_{a}^{(n)} \right)^{2}r{\mathbb{d}r}{\mathbb{d}\theta}}}}}}}},$where the proportionality refers to the stationary coefficient ofa_(n)(z,t). Strictly speaking, the set of acoustic modes is complete butnot orthogonal [see E. Peral and A. Yariv, IEEE J. Quantum Electronics,vol. 35, pp. 1185–1195 (1999)], but the assumption of orthogonality issupported by the good agreement between the predicted and measured SBSthresholds of the optical fibers disclosed herein.

The refractive index change caused by the acoustic field is proportionalto the density change [R. W. Boyd, Nonlinear Optics, 2nd edition,Academic Press 2003, p. 404], i.e. Δn˜{tilde under (ρ)}−ρ₀. Therefore

${\Delta\; n} \sim {\frac{\int{\int{f_{o}^{2}f_{a}^{(n)}r{\mathbb{d}r}{\mathbb{d}\theta}}}}{\int{\int{f_{o}^{2}r{\mathbb{d}r}{\mathbb{d}\theta}{\int{\int{\left( f_{a}^{(n)} \right)^{2}r{\mathbb{d}r}{\mathbb{d}\theta}}}}}}}{f_{a}^{(n)}.}}$

From the standard perturbation theory, the change in the propagationconstant is [G. P. Agrawal, Nonlinear Fiber Optics, 3d edition, AcademicPress, 2001, p. 43]:

${\Delta\beta} \sim \frac{\int{\int{\Delta\;{nf}_{o}^{2}r{\mathbb{d}r}{\mathbb{d}\theta}}}}{\int{\int{f_{o}^{2}r{\mathbb{d}r}{\mathbb{d}\theta}}}}$Then in analogy to the optical effective area, the acousto-opticeffective area (AOEA) is introduced which depends on the overlap betweenthe optical mode and the nth acoustic mode:

$A_{eff}^{{a - o},{(n)}} = {\left\lbrack \frac{\int{\int{f_{o}^{2}r{\mathbb{d}r}{\mathbb{d}\theta}}}}{\int{\int{f_{o}^{2}f_{a}^{(n)}r{\mathbb{d}r}{\mathbb{d}\theta}}}} \right\rbrack^{2}{\int{\int{\left( f_{a}^{(n)} \right)^{2}r{\mathbb{d}r}{\mathbb{d}\theta}}}}}$Since the imaginary part of Δβ represents the Brillouin gain, it isinversely proportional to the AOEA. Therefore, a small overlap betweenoptical and acoustic fields leads to a large acousto-optic effectivearea and consequently small Brillouin gain for the respective acousticmode.

For input signal at 1550 nm, values of the acousto-optic effective areaof the optical fiber are calculated for the respective acoustic mode.

We have designed waveguides with robust optical properties and largeacousto-optic effective areas. Measurements of a large number offabricated optical fibers have verified the connection betweenacousto-optic effective area and the SBS gain factor.

An optical waveguide fiber which is optically single-moded at aparticular wavelength may be multi-moded acoustically at the sameoptical wavelength because the acoustic wavelength corresponding toBrillouin frequency is of the order of 0.55 microns, which is quitesmall compared to typical optical waveguide fiber dimensions. In thecase of spontaneous Brillouin scattering at relatively low launchpowers, the incident optical field is Brillouin scattered by each of theacoustic modes and Brillouin gain spectrum shows peaks corresponding tooptical field interaction with each of the acoustic modes. At relativelyhigh launch powers, the SBS threshold is exceeded, and one of theacoustic modes typically becomes dominant while the other acoustic modesdo not survive the mode competition, leading to the onset of stimulatedBrillouin scattering.

As coupling between the optical mode field and the acoustic modefield(s) increases, more optical power is undesirably reflected oppositeto the direction of optical signal transmission.

As disclosed herein, the coupling between the optical and acousticalmodes is preferably reduced via the refractive index profiles of theoptical fiber disclosed herein. Preferably, the optical mode fieldremains extended while acoustical fields become more tightly confined toreduce overlap between the optical mode field and the acoustical fields.

The optical fiber disclosed herein tends to pull the mode field of thedominant acoustic mode field (typically L₀₁) in toward the centerline ofthe optical fiber, resulting in reduced coupling between the acousticand optical fields. The optical fiber disclosed herein also preferablytends to pull the mode field of the next dominant acoustic mode field(typically L₀₂) in toward the centerline of the optical fiber, resultingin reduced coupling between this next dominant acoustic mode field andthe optical field.

Generally, the acoustic field in an optical fiber is more localized(typically much more localized) near the centerline of the fibercompared to the optical field. Accordingly, the behavior of the acousticfield is greatly affected in the central portion of the core of theoptical fiber, for example by density variations in the central 2 micronradial region of the optical fiber and consequently within therefractive index profile of the core of the fiber.

We have found that, in order to achieve high SBS threshold opticalfiber, the minimum acousto-optic area of the optical fiber should be aslarge as possible. In various embodiments, we have also found that theacousto-optic area of the dominant acoustic mode (typically L₀₁) and theacousto-optic area of the second most dominant acoustic mode (typicallyL₀₂) generally should be as close to one another in magnitude aspossible. Without being limited by any particular theory, the relativecloseness in value of the acousto-optic areas of these two modes appearsto allow division of the optical-acoustic coupling between the twoacoustic modes, thereby reducing coupling overall synergistically in amanner not possible by simply having one acousto-optic area which isvery large in magnitude while the other acousto-optic area is muchsmaller than the very large acousto-optic area. Also, the optical fieldmay couple to more than two acoustic modes, thereby providing additionalpaths for the dissipation of the reflected signal.

The Brillouin frequency for optical fiber as disclosed herein ispreferably between about 10 to 12 GHz.

The optical fiber disclosed herein comprises a core and a cladding layer(or cladding) surrounding and directly adjacent the core. The claddinghas a refractive index profile, Δ_(CLAD)(r). Preferably, Δ_(CLAD)(r)=0throughout the cladding. The core comprises a refractive index profile,Δ_(CORE)(r). The core has a maximum relative refractive index, Δ_(MAX),in %, occurring at a radius r_(ΔMAX). In preferred embodiments, the coreis comprised of a plurality of core portions, each having respectiverefractive index profiles, for example Δ_(CORE1)(r), Δ_(CORE2)(r), andso on. Each core portion may have a maximum relative refractive indexfor that core portion, i.e. a local maximum, reported in %, for exampleΔ_(1MAX) for a first core portion, Δ_(2MAX) for a second portion, and soon. Likewise, a core portion may have a minimum relative refractiveindex, such as Δ_(2MIN), etc. A maximum or minimum relative refractiveindex may occur at a particular radius, such as r_(Δ1MIN2) or r_(Δ2MIN)and so on. For the embodiments disclosed herein, the core is definedherein to end at a radius r_(CORE).

We have found that a higher dopant concentration at or near the opticalfiber centerline, and in particular in a central portion of the core ofthe optical fiber, forces the acoustic modes to be more tightlyconfined.

Preferably, the core is comprised of silica doped with germanium, i.e.germania doped silica. Doping of the core, and in particular the centralportion of the core, advantageously reduces sound velocity in theoptical fiber core relative to its cladding, resulting in total internalreflection of the acoustic field. Dopants other than germanium, singlyor in combination, may be employed within the core, and particularly ator near the centerline, of the optical fiber disclosed herein to obtainthe desired refractive index and density. Although high refractive indexvalues (or high levels of updopant) tend to bring the acoustical modefield toward the centerline, such values also tend to decrease theoptical effective area In preferred embodiments, the core of the opticalfiber disclosed herein has a non-negative refractive index profile, morepreferably a positive refractive index profile, wherein the core issurrounded by and directly adjacent to a cladding layer.

Preferably, the refractive index profile of the optical fiber disclosedherein is non-negative from the centerline to the outer radius of thecore, r_(CORE). In preferred embodiments, the optical fiber contains noindex-decreasing dopants in the core.

1^(st) Set of Preferred Embodiments

Table 1 lists an illustrative first set of preferred embodiments,Examples 1–3. FIG. 1 shows the corresponding refractive index profilesof Examples 1–3 in curves 1–3, respectively.

TABLE 1 Example: Ex 1 Ex 2 Ex 3 Dispersion at ps/nm-km 6.7 7.4 4.77 1550nm Dispersion at ps/nm-km 13.1 13.6 11.1 1625 nm Dispersion Slopeps/nm²-km 0.089 0.086 0.086 at 1550 nm Kappa at 1550 nm nm 75 86 55MFD1550 μm 10.16 10.08 9.87 Attenuation at dB/km 0.193 0.192 0.194 1550nm Pin Array at dB 7.2 6.9 9.6 1550 nm LP11 cutoff nm 1684 1608 1654(theoretical) Fiber Cutoff nm 1380 1330 1325 Cabled Cutoff nm <1300<1300 <1300 Zero Dispersion nm 1477 1468 1496 Wavelength A_(eff) at 1550nm μm² 77.6 76.0 73.1 AOEA_(L01) μm² 213 215 174 AOEA_(L02) μm² 218 212210 AOEA_(L03) μm² 440 412 507 AOEA_(L04) μm² 2307 1712 3103 AOEA_(L05)μm² 2107 1592 2059 AOEA_(MIN) μm² 213 212 174 Ratio: — 0.98 1.01 0.82AOEA_(L01)/AOEA_(L02) SBS Threshold dBm 10.6 10.6 9.7 (absolute) SBSThreshold dB 3.8 3.8 2.9 Improvement over SMF-28 ™ optical fiber Δ(r =0) % 0.83 0.84 0.80 Δ₁(r = 1) % 0.59 0.58 0.59 Δ(0) − Δ(1) % 0.24 0.260.21 Δ_(MAX) % 0.83 0.84 0.80 r_(ΔMAX) μm 0.0 0.0 0.0 Δ(r = 1.5 μm) %0.42 0.42 0.47 Δ(r = 2 μm) % 0.34 0.33 0.36 Δ(r = 2.5 μm) % 0.26 0.250.26 Δ(r = 3 μm) % 0.17 0.17 0.16 Δ(r = 3.5 μm) % 0.09 0.10 0.07 Δ(r = 4μm) % 0.03 0.06 0.016 Δ(r = 4.5 μm) % 0.075 0.11 0.03 Δ(r = 5 μm) % 0.140.14 0.11 Δ(r = 5.5 μm) % 0.15 0.14 0.13 Δ(r = 6 μm) % 0.15 0.14 0.13Δ(r = 6.5 μm) % 0.15 0.14 0.13 Δ(r = 7 μm) % 0.15 0.14 0.13 Δ(r = 7.5μm) % 0.15 0.14 0.13 Δ(r = 8 μm) % 0.14 0.09 0.13 Δ(r = 8.5 μm) % 0.070.02 0.13 Δ(r = 9 μm) % 0.01 0.00 0.06 Δ(r = 9.5 μm) % 0.00 0.00 0.00 r₂μm 3.45 3.45 3.3 r₃ μm 4.65 4.4 4.9 w_(MOAT) μm 1.2 0.95 1.6 r_(MOAT) μm4.05 3.9 4.1 r_(CORE) μm 9.05 8.75 9.4 Δ_(2MIN) % 0.03 0.06 0.013r_(ΔMIN2) μm 4.1 4.0 4.2 Δ_(3MAX) % 0.15 0.14 0.13 r₄ μm 8.3 7.95 8.8Ring Width μm 3.7 3.6 3.9 Ring Midpoint μm 6.5 6.2 6.9

The optical fibers illustrated by Examples 1–3 have an optical modeeffective area at 1550 nm which is less than about 80 μm², a firstacoustical mode L₀₁ having a first acousto-optic effective area,AOEA_(L01), which is not less than 150 μm²; and a second acoustical modeL₀₂ having a second acousto-optic effective area, AOEA_(L02), which isnot less than 150 μm², wherein 0.4<AOEA_(L01)/AOEA_(L02)<2.5.Preferably, the optical mode effective area at 1550 nm is between about50 and 80 μm², more preferably between about 60 and 80 μm². In somepreferred embodiments, the optical mode effective area at 1550 nm isbetween about 70 and 80 μm². Preferably, the fiber cutoff is less thanabout 1450 nm. Cabled cutoff is about 100 to 250 nm lower than the fibercutoff for these fibers. Preferably, cabled cutoff is less than about1300 nm, more preferably between about 1100 and 1300 nm. In somepreferred embodiments, the core may comprise a relative refractive indexprofile having a so-called centerline dip which may occur as a result ofone or more optical fiber manufacturing techniques. However, thecenterline dip in any of the refractive index profiles disclosed hereinis optional. The core comprises a first portion extending from thecenterline to a radius of 1 μm. The core has a maximum relativerefractive index Δ_(MAX) (in percent) less than 1.0%, more preferablyless than 0.9%, occurring at a radius r_(ΔMAX). In preferredembodiments, Δ_(MAX) is greater than 0.7% and less than 0.9%. In otherpreferred embodiments, Δ_(MAX) is greater than 0.75% and less than 0.9%.In still other preferred embodiments, Δ_(MAX) is greater than 0.75% andless than 0.85%. The relative refractive index Δ_(CORE1)(r) ispreferably greater than 0.4% and less than 1.0%, more preferably greaterthan 0.5% and less than 0.9% for all radii between r_(ΔMAX) and r=1 μm.The core further comprises a second portion surrounding and directlyadjacent to the first portion, the second portion having a relativerefractive index which decreases with increasing radius, the secondportion extending to a radius r₂ where the relative refractive indexreaches a minimum relative refractive index Δ_(2MIN) of 0.1%. Δ_(MAX)occurs in the first or second portions, preferably in the first portion.Preferably, the second portion extends to a radius of between 2.5 μm and4.5 μm, more preferably between 3 μm and 4 μm. In some preferredembodiments, the second portion extends to a radius of about 3.5 μm. Inthe second portion, Δ_(CORE2)(r) is greater than 0.1% and less than0.8%, more preferably greater than 0.1% and less than 0.7%, throughoutthe second portion. In preferred embodiments, Δ_(CORE2)(r) is greaterthan 0.35 and less than 0.7% from r=1 to r=1.5 μm, and in preferredembodiments, Δ_(CORE2)(r) is greater than about 0.2% and less than 0.6%from r=1.5 to r=2.5 μm. In preferred embodiments, the difference betweenΔ_(MAX) and Δ_(2MIN) (Δ_(MAX)−Δ_(2MIN)) is greater than 0.6%. In otherpreferred embodiments, (Δ_(MAX)−Δ_(2MIN))>0.7%. The first and secondportions of the core together form a central region of the core, orcentral segment 10. The core comprises a third portion surrounding anddirectly adjacent to the central segment 10. The third portion extendsto a radius r₃ where Δ_(CORE3)(r) reaches 0.1%, where r₃>r₂. Preferably,r₃ is between 3.5 and 5.5 μm, more preferably between 4 and 5 μm. Thethird portion comprises a minimum relative refractive index Δ_(3MIN)greater than 0% and less than 0.1%, more preferably greater than 0.01%and less than 0.1%. In preferred embodiments, the difference betweenΔ_(MAX) and Δ_(3MIN)(Δ_(MAX)−Δ_(3MIN)) is greater than 0.75%. In otherpreferred embodiments, (Δ_(MAX)−Δ_(MIN))>0.8%. The third portion forms amoat segment 12. Preferably, the width of the moat segmentw_(MOAT)=(r₃−r₂) is less than about 1.5 μm. In preferred embodiments,the moat segment has midpoint r_(MOAT) of about 4 μm. The core comprisesa fourth portion surrounding and directly adjacent to the third portion.The fourth portion extends to a radius r_(CORE) of between 8 and 12 μm,more preferably between 8 and 10 μm where Δ_(CORE)(r) reaches 0%, i.e.directly adjacent to the cladding 200. The fourth portion comprises amaximum relative refractive index Δ_(4MAX) (in percent) which is greaterthan Δ_(3MIN) and less than Δ_(MAX). Preferably, the difference betweenΔ_(4MAX) and Δ_(3MIN) (Δ_(4MAX)−Δ_(3MIN)) is greater than 0.05%.Preferably, Δ_(4MAX) is less than 0.2%, more preferably less than 0.15%,even more preferably greater than 0.1% and less than 0.15%. The fourthportion forms a ring segment 14 directly adjacent to the moat segment.The ring segment preferably has a width of greater than 2 μm, morepreferably greater than 3 μm, even more preferably between 3 and 4 μm,and preferably having a midpoint located at a radius of between 6 and 8μm, more preferably between 6 and 7 μm. The width is defined to extendfrom r₃ to a radius r₄ where Δ_(CORE4)(r) reaches 0.1% and r₄>r₃.Preferably, Δ_(CORE4)(r) is less than 0.1% for all radii greater than 9μm. Preferably, the cladding 200 surrounds and is directly adjacent tothe ring segment 14. Preferably, the relative refractive index is lessthan 0.03% for all radii between 10 and 25 μm. Preferably, the core endsand the cladding 200 begins at r_(CORE) of greater than about 8 μm, morepreferably between 8 and 15 μm. The core thus comprises three segments,a central segment 10 contactingly surrounded by a moat segment 12, themoat segment being contactingly surrounded by a ring segment 14. FIG. 1shows r₂, r₃, and r₄ corresponding to curve 3 for illustration purposes.

In preferred embodiments, optical fiber such as those illustrated byExamples 1–3 disclosed herein preferably have: a positive dispersion at1550 nm having an absolute magnitude of less than 7.5 ps/nm-km, morepreferably between 1 and 7.5 ps/nm-km, even more preferably between 1and 7 ps/nm-km; a dispersion slope at 1550 nm of less than 0.09ps/nm²-km, more preferably between 0.07 and 0.09 ps/nm²-km; dispersionzero less than 1500 nm; an optical effective area at 1550 nm less than80 μm, more preferably between 50 and 80 μm², even more preferablybetween 60 μm² and 80 μm²; an optical mode field diameter at 1550 nm ofless than 10.5 μm; and pin array bend loss at 1550 nm less than 12 dB.Preferably, attenuation at 1550 nm is less than 0.21 dB/km, morepreferably less than 0.20 dB/km.

Preferably, the relative refractive index of the second portionmonotonically decreases for increasing radius.

In some preferred embodiments, such as Example 3, the second portion hasan alpha less than 1.

Preferably, the part of the relative refractive index that includesΔ_(4MAX) in the ring segment 14 of the core is relatively flat, i.e. ofrelatively constant value. Preferably, the absolute magnitude of thedifference in Δ(r) between any radii between r=5.5 μm and r=7.5 μm isless than 0.07%, more preferably less than 0.05%.

In preferred embodiments, the change in the relative refractive indexbetween Δ_(3MIN) and Δ_(4MAX) is gradual. Preferably, the absolute valueof the change in relative refractive index is less than 0.3%/μm, morepreferably by less than 0.2%/μm.

In preferred embodiments, the change in the relative refractive index atthe outer part of the ring segment 14 is gradual. Preferably, theabsolute value of the change in relative refractive index is less than0.3%/μm, more preferably by less than 0.2%/μm.

A_(L01) may be greater than A_(L02), or A_(L02) may be greater thanA_(L01), or A_(L01) may be substantially equal to A_(L02). Preferably,A_(L01) and A_(L02) are both greater than 150 μm², more preferablygreater than 175 μm², and even more preferably greater than 200 μm².

In preferred embodiments, 0.5<AOEA_(L01)/AOEA_(L02)<2. In otherpreferred embodiments, 0.6<AOEA_(L01)/AOEA_(L02)<1.5.

Cladding 200, or a cladding layer, shown in FIG. 1 where Δ_(CLAD)=0,surrounds and is directly adjacent the outer region of the core.

Examples 1–3 can thus be described as having a core comprising a centralsegment 10, a moat segment 12 surrounding and directly adjacent to thecentral segment, and a ring segment 14 surrounding and directly adjacentto the moat segment. The core preferably has an entirely positiverefractive index and Δ_(MAX)>Δ_(4MAX)>Δ_(2MIN)>Δ_(3MIN)>0. Preferably,the relative refractive index profile of the ring segment 14 comprises asubstantially flat part, and more preferably, a substantial part of thering segment has a substantially flat relative refractive index profileΔ_(CORE4)(r).

2^(nd) Set of Preferred Embodiments

Table 2 lists an illustrative second set of preferred embodiments,Examples 4–5. FIG. 2 shows the corresponding refractive index profilesof Examples 4–5 in curves 4–5, respectively.

TABLE 2 Example: Ex 4 Ex 5 Dispersion at 1550 nm ps/nm-km 5.3 5.3Dispersion at 1625 nm ps/nm-km 11.6 11.1 Dispersion Slope at ps/nm²-km0.086 0.077 1550 nm Kappa at 1550 nm nm 62 69 MFD1550 μm 10.1 9.79Attenuation at 1550 nm dB/km 0.194 0.193 Pin Array at 1550 nm dB 11.510.2 LP11 cutoff (theoretical) nm 1698 1681 Fiber Cutoff nm 1367 1320Cabled Cutoff nm <1300 <1300 Zero Dispersion nm 1486 1481 WavelengthA_(eff) at 1550 nm μm² 77.0 71.3 AOEA_(L01) μm² 185 173 AOEA_(L02) μm²221 194 AOEA_(L03) μm² 561 512 AOEA_(L04) μm² 5066 11026 AOEA_(L05) μm²7936 1323 AOEA_(MIN) μm² 185 173 Ratio: AOEA_(L01)/AOEA_(L02) 0.84 0.89SBS Threshold dBm 10.0 9.7 (absolute) SBS Threshold dB 3.2 2.9Improvement over SMF-28 ™ optical fiber Δ(r = 0) % 0.78 0.77 Δ₁(r = 1) %0.56 0.56 Δ(0) − Δ(1) % 0.22 0.25 Δ_(MAX) % 0.78 0.77 r_(ΔMAX) μm 0.00.0 Δ(r = 1.5 μm) % 0.46 0.45 Δ(r = 2 μm) % 0.36 0.36 Δ(r = 2.5 μm) %0.26 0.28 Δ(r = 3 μm) % 0.17 0.20 Δ(r = 3.5 μm) % 0.09 0.13 Δ(r = 4 μm)% 0.02 0.06 Δ(r = 4.5 μm) % 0.01 0.02 Δ(r = 5 μm) % 0.01 0.01 Δ(r = 5.5μm) % 0.09 0.01 Δ(r = 6 μm) % 0.16 0.05 Δ(r = 6.5 μm) % 0.16 0.14 Δ(r =7 μm) % 0.16 0.14 Δ(r = 7.5 μm) % 0.16 0.14 Δ(r = 8 μm) % 0.16 0.14 Δ(r= 8.5 μm) % 0.14 0.14 Δ(r = 9 μm) % 0.03 0.13 Δ(r = 9.5 μm) % 0.00 0.02r_(CORE) μm 9.15 9.7 r₂ μm 3.75 4.1 r₃ μm 5.35 6 w_(MOAT) μm 1.6 1.9r_(MOAT) μm 4.55 5.05 Δ_(2min) % 0.01 0.01 r_(ΔMIN2) μm 4.7 5.15Δ_(3MAX) % 0.16 0.14 r₄ μm 8.8 9.4 Ring Width μm 3.45 3.4 Ring Midpointμm 7.1 7.7

The optical fibers illustrated by Examples 4–5 have an optical modeeffective area at 1550 nm which is less than about 80 m², a firstacoustical mode L₀₁ having a first acousto-optic effective area,AOEA_(L01), which is not less than 150 μm²; and a second acoustical modeL₀₂ having a second acousto-optic effective area, AOEA_(L02), which isnot less than 150 μm², wherein 0.4<AOEA_(L01)/AOEA_(L02)<2.5.Preferably, the optical mode effective area at 1550 nm is between about50 and 80 μm², more preferably between about 60 and 80 μm². In somepreferred embodiments, the optical mode effective area at 1550 nm isbetween about 70 and 80 μm². Preferably, the fiber cutoff is less thanabout 1450 nm. Cabled cutoff is about 100 to 250 nm lower than the fibercutoff for these fibers. Preferably, cabled cutoff is less than about1300 nm, more preferably between about 1100 and 1300 nm. In somepreferred embodiments, the core may comprise a relative refractive indexprofile having a so-called centerline dip which may occur as a result ofone or more optical fiber manufacturing techniques. However, thecenterline dip in any of the refractive index profiles disclosed hereinis optional. The core comprises a first portion extending from thecenterline to a radius of 1 μm. The core has a maximum relativerefractive index Δ_(MAX) (in percent) less than 1.0%, more preferablyless than 0.9%, occurring at a radius r_(ΔMAX). In preferredembodiments, Δ_(MAX) is greater than 0.7% and less than 0.9%. In otherpreferred embodiments, Δ_(MAX) is greater than 0.75% and less than 0.9%.In still other preferred embodiments, Δ_(MAX) is greater than 0.75% andless than 0.85%. The relative refractive index Δ_(CORE1)(r) ispreferably greater than 0.4% and less than 1.0%, more preferably greaterthan 0.5% and less than 0.9% for all radii between r_(ΔMAX) and r=1 μm.The core further comprises a second portion surrounding and directlyadjacent to the first portion, the second portion having a relativerefractive index which decreases with increasing radius, the secondportion extending to a radius r₂ where the relative refractive indexreaches a minimum relative refractive index Δ_(2MIN) 0.05%. Δ_(MAX)occurs in the first or second portions, preferably in the first portion.Preferably, the second portion extends to a radius of between 3 μm and 5μm, more preferably between 3.5 μm and 4.5 μm. In preferred embodiments,the second portion extends to a radius of about 4 μm. In the secondportion, Δ_(CORE)(r) is greater than 0.05% and less than 0.8%, morepreferably greater than 0.05% and less than 0.7%, throughout the secondportion. In preferred embodiments, Δ_(CORE2)(r) is greater than 0.35 andless than 0.7% from r=1 to r=1.5 μm, and in preferred embodiments,Δ_(CORE2)(r) is greater than about 0.2% and less than 0.6% from r=1.5 tor=2.5 μm. Preferably, the second portion has an alpha less than 1. Inpreferred embodiments, (Δ_(MAX)−Δ_(2MIN))>0.7%. The first and secondportions of the core together form a central region of the core, orcentral segment 10. The core comprises a third portion surrounding anddirectly adjacent to the central segment 10. The third portion extendsto a radius r₃ where Δ_(CORE3)(r) reaches 0.05%, where r₃>r₂.Preferably, r₃ is between 4.5 and 7 μm, more preferably between 5 and6.5 μm. The third portion comprises a minimum relative refractive indexΔ_(3MIN) greater than 0% and less than 0.05%, more preferably greaterthan 0.01% and less than 0.05%. In preferred embodiments, the differencebetween Δ_(MAX) and Δ_(3MIN) (Δ_(MAX)−Δ_(3MIN)) is greater than 0.6%. Inother preferred embodiments, (Δ_(MAX)−Δ_(3MIN))>0.7%. The third portionforms a moat segment 12. Preferably, the width w_(MOAT)=(r₃−r₂) of themoat segment 12 is greater than about 1.5 μm. In preferred embodiments,the moat segment 12 has a midpoint r_(MOAT) at greater than about 5 μm.The core comprises a fourth portion surrounding and directly adjacent tothe moat segment 12. The fourth portion extends to a radius r_(CORE) ofbetween 8 and 12 μm, more preferably between 9 and 11 μm whereΔ_(CORE)(r) reaches 0%, i.e. directly adjacent to the cladding 200. Thefourth portion comprises a maximum relative refractive index Δ_(4MAX)(inpercent) which is greater than Δ_(3MIN) and less than Δ_(MAX).Preferably, the difference between Δ_(4MAX) andΔ_(3MIN)(Δ_(4MAX)−Δ_(3MIN)) is greater than 0.05%, more preferablygreater than 0.1%. Preferably, Δ_(4MAX) is less than 0.3%, morepreferably less than 0.25%, even more preferably greater than 0.1% andless than 0.2%. The fourth portion forms a ring segment 14 directlyadjacent to the moat segment 12. The ring segment 14 preferably has awidth of less than 4 μm, more preferably between 3 and 4 μm, andpreferably having a midpoint located at a radius of between 6 and 8 μm.The width is defined to extend from r₃ to a radius r₄ where Δ_(CORE4)(r)reaches 0.05% and r₄>r₃. Preferably, Δ_(CORE4)(r) is less than 0.1% forall radii greater than 9.5 μm. Preferably, the cladding 200 surroundsand is directly adjacent to the ring segment 14. The core thus comprisesthree segments, a central segment 10 contactingly surrounded by a moatsegment 12, the moat segment being contactingly surrounded by a ringsegment 14. FIG. 2 shows r₂, r₃, and r₄ corresponding to curve 4 forillustration purposes.

In preferred embodiments, optical fiber such as those illustrated byExamples 4–5 disclosed herein preferably have: a positive dispersion at1550 nm having an absolute magnitude of less than 7.5 ps/nm-km, morepreferably between 1 and 7.5 ps/nm-km, even more preferably between 1and 7 ps/nm-km, still more preferably between 3 and 7 ps/nm-km; adispersion slope at 1550 nm of less than 0.09 ps/nm²-km, more preferablybetween 0.07 and 0.09 ps/nm²-km; dispersion zero less than 1500 nm; anoptical effective area at 1550 nm less than 80 μm², more preferablybetween 50 and 80 μm², even more preferably between 60 μm² and 80 μm²;an optical mode field diameter at 1550 nm of less than 10.5 μm; and pinarray bend loss at 1550 nm less than 12 dB. Preferably, attenuation at1550 nm is less than 0.21 dB/km, more preferably less than 0.20 dB/km.

Preferably, the relative refractive index of the second portionmonotonically decreases for increasing radius.

Preferably, the part of the relative refractive index that includesΔ_(4MAX) in the ring segment 14 of the core is relatively flat, i.e. ofrelatively constant value. Preferably, the absolute magnitude of thedifference in Δ(r) between any radii between r=7 μm and r=8 μm is lessthan 0.07%, more preferably less than 0.05%.

In preferred embodiments, the change in the relative refractive indexbetween Δ_(3MIN) and Δ_(4MAX) is gradual. Preferably, the absolute valueof the change in relative refractive index is less than 0.3%/μm, morepreferably by less than 0.2%/μm.

In preferred embodiments, the change in the relative refractive index atthe outer part of the ring segment 14 is gradual. Preferably, theabsolute value of the change in relative refractive index is less than0.3%/μm, more preferably by less than 0.2%/μm.

A_(L01) may be greater than A_(L02), or A_(L02) may be greater thanA_(L01) or A_(L02) may be substantially equal to A_(L02). Preferably,A_(L01) and A_(L02) are both greater than 150 μm², more preferablygreater than 170 μm², and even more preferably greater than 190 μm².

In preferred embodiments, 0.5<AOEA_(L01)/AOEA_(L02)<2. In otherpreferred embodiments, 0.6<AOEA_(L01)/AOEA_(L02)<1.5.

Cladding, or a cladding layer, shown in FIG. 2 where Δ_(CLAD)=0,surrounds and is directly adjacent the outer region of the core.

Examples 4–5 can thus be described as having a core comprising a centralsegment 10, a moat segment 12 surrounding and directly adjacent to thecentral segment, and a ring segment 14 surrounding and directly adjacentto the moat segment. The core preferably has a positive refractive indexand Δ_(MAX)>Δ_(4MAX)>Δ_(2MIN)>Δ_(3MIN)>0. Preferably, the relativerefractive index profile of the moat segment 12 comprises asubstantially flat part. Preferably, the relative refractive indexprofile of the ring segment 14 comprises a substantially flat part, andmore preferably, a substantial part of the ring segment has asubstantially flat relative refractive index profile Δ_(CORE4)(r).

3^(rd) Set of Preferred Embodiments

Table 3 lists an illustrative third set of preferred embodiments,Examples 6–8. FIG. 3 shows the corresponding refractive index profilesof Examples 6–8 in curves 6–8, respectively.

TABLE 3 Example: Ex 6 Ex 7 Ex 8 Dispersion at ps/nm-km −4.0 −2.7 −2.21550 nm Dispersion at ps/nm-km −0.7 0.6 1.0 1575 nm Dispersion atps/nm-km 2.4 3.8 4.1 1600 nm Dispersion at ps/nm-km 5.4 6.9 7.1 1625 nmDispersion Slope ps/nm²-km 0.134 0.130 0.131 at 1550 nm MFD1550 μm 9.369.56 9.61 Attenuation at dB/km 0.206 0.206 0.204 1550 nm Pin Array at dB6.2 6.1 6.5 1550 nm LP11 cutoff nm 1714 1738 1749 (theoretical) FiberCutoff nm 1432 1465 1465 Cabled Cutoff nm <1400 <1400 <1400 ZeroDispersion nm 1581 1570 1567 Wavelength A_(eff) at 1550 nm μm² 70.7 74.374.6 AOEA_(L01) μm² 172 202 198 AOEA_(L02) μm² 357 355 355 AOEA_(MIN)μm² 172 202 198 Ratio: 0.48 0.57 0.56 AOEA_(L01)/AOEA_(L02) SBSThreshold dBm 9.7 10.4 10.3 (absolute) SBS Threshold dB 2.9 3.6 3.5Improvement over SMF-28 ™ optical fiber Δ(r = 0) % 1.09 1.17 1.12 Δ(r =0.5) % 1.05 1.11 1.07 Δ₁(r = 1) % 0.80 0.76 0.76 Δ(0) − Δ(1) % 0.29 0.410.36 Δ_(MAX) % 1.09 1.17 1.12 r_(ΔMAX) μm 0.0 0.0 0.0 Δ(r = 1.5 μm) %0.37 0.33 0.34 Δ(r = 2 μm) % 0.18 0.18 0.19 Δ(r = 2.5 μm) % 0.13 0.140.15 Δ(r = 3 μm) % 0.10 0.11 0.12 Δ(r = 3.5 μm) % 0.07 0.08 0.09 Δ(r = 4μm) % 0.05 0.06 0.06 Δ(r = 4.5 μm) % 0.06 0.07 0.06 Δ(r = 5 μm) % 0.120.14 0.11 Δ(r = 5.5 μm) % 0.22 0.23 0.20 Δ(r = 6 μm) % 0.25 0.25 0.24Δ(r = 6.5 μm) % 0.25 0.25 0.25 Δ(r = 7 μm) % 0.22 0.22 0.23 Δ(r = 7.5μm) % 0.11 0.11 0.15 Δ(r = 8 μm) % 0.02 0.02 0.04 Δ(r = 8.5 μm) % 0.000.00 0.00 r₂ μm 2.2 2.3 2.45 r₃ μm 5.1 5.05 5.2 w_(MOAT) μm 2.9 2.752.75 r_(MOAT) μm 3.65 3.7 3.8 r_(CORE) μm 8.2 8.3 8.4 Δ_(2MIN) % 0.050.06 0.06 r_(ΔMIN2) μm 4.15 4.2 4.3 Δ_(3MAX) % 0.25 0.25 0.25 r_(3MAX)μm 6.3 6.25 6.4 r₄ μm 7.35 7.35 7.5 Ring Width μm 2.3 2.3 2.3 RingMidpoint μm 6.2 6.2 6.35

The optical fibers illustrated by Examples 6–8 have an optical modeeffective area at 1550 nm which is less than about 80 μm², a firstacoustical mode L₀₁ having a first acousto-optic effective area,AOEA_(L01), which is not less than 150 μm²; and a second acoustical modeL₀₂ having a second acousto-optic effective area, AOEA_(L02), which isnot less than 150 μm², wherein 0.4<AOEA_(L01)/AOEA_(L02)<2.5.Preferably, the optical mode effective area at 1550 nm is between about50 and 80 μm², more preferably between about 60 and 80 μm². In somepreferred embodiments, the optical mode effective area at 1550 nm isbetween about 70 and 80 μm². Preferably, the fiber cutoff is less thanabout 1500 nm. Cabled cutoff is about 100 to 250 nm lower than the fibercutoff for these fibers. Preferably, cabled cutoff is less than about1400 nm, more preferably between about 1200 and 1300 nm. In somepreferred embodiments, the core may comprise a relative refractive indexprofile having a so-called centerline dip which may occur as a result ofone or more optical fiber manufacturing techniques. However, thecenterline dip in any of the refractive index profiles disclosed hereinis optional. The core comprises a first portion extending from thecenterline to a radius of 1 μm. The core has a maximum relative relativerefractive index Δ_(MAX) (in percent) greater than 0.9%, more preferablygreater than 1.0%, occurring at a radius r_(ΔMAX). In preferredembodiments, Δ_(MAX) is greater than 1.0% and less than 1.3%. Therelative refractive index Δ_(CORE1)(r) is preferably greater than 0.4%and less than 1.3%, more preferably greater than 0.5% and less than 1.2%for all radii between r_(ΔMAX) and r=1 μm. The core further comprises asecond portion surrounding and directly adjacent to the first portion,the second portion having a relative refractive index which decreaseswith increasing radius, the second portion extending to a radius r₂where the relative refractive index reaches a minimum relativerefractive index Δ_(2MIN) of 0.15%. Δ_(MAX) occurs in the first orsecond portions, preferably in the first portion. Preferably, the secondportion extends to a radius of between 1.5 μm and 3 μm, more preferablybetween 1.5 μm and 2.5 μm. In preferred embodiments, the second portionextends to a radius of about 2 μm. Δ_(CORE2)(r) is between 0.15% and1.0%, more preferably between 0.15% and 0.9%, throughout the secondportion. In preferred embodiments, Δ_(CORE2)(r) is greater than 0.2% andless than 0.9% from r=1 to r=1.5 μm, and in preferred embodiments,Δ_(CORE2)(r) is greater than about 0.1% and less than 0.5% from r=1.5 tor=2 μm. In preferred embodiments, (Δ_(MAX)−Δ_(2MIN))>0.85%. Together,the first and second portions of the core form a central segment 10 ofthe core. The core comprises a third portion surrounding and directlyadjacent to the second portion. The third portion extends to a radius r₃where Δ_(CORE3)(r) reaches 0.15%, where r₃>r₂. Preferably, r₃ is between4.5 and 7 μm, more preferably between 5 and 6.5 μm. The third portioncomprises a minimum relative refractive index Δ_(3MIN) greater than 0%and less than 0.1%, more preferably greater than 0.01% and less than0.1%. In preferred embodiments, the difference between Δ_(MAX) andΔ_(3MIN) (Δ_(MAX)−Δ_(3MIN)) is greater than 0.9%. In other preferredembodiments, (Δ_(MAX)−Δ_(3MIN))>1%. The third portion forms a moatsegment 12. In preferred embodiments, the width w_(MOAT)=(r₃−r₂) of themoat segment 12 is between 2.5 and 3 μm. In preferred embodiments, themoat segment 12 has a midpoint r_(MOAT) at about 3 to 4 μm. The corecomprises a fourth portion surrounding and directly adjacent to thethird portion. The fourth portion extends to a radius r_(CORE) ofbetween 8 and 12 μm, more preferably between 9 and 11 μm whereΔ_(CORE)(r) reaches 0%. The fourth portion comprises a maximum relativerefractive index Δ_(4MAX) (in percent) which is greater than Δ_(3MIN)and less than Δ_(MAX). Preferably, the difference between Δ_(4MAX) andΔ_(3MIN) (Δ_(4MAX)−Δ_(3MIN)) is greater than 0.1%, more preferablygreater than 0.15%. Preferably, Δ_(4MAX) is less than 0.4%, morepreferably less than 0.3%, even more preferably greater than 0.15% andless than 0.3%. The fourth portion forms a ring segment 14 directlyadjacent to the moat segment 12. The ring segment 14 preferably has awidth of greater than 2 μm, more preferably between 2 and 3 μm, andpreferably having a midpoint located at a radius of between 6 and 7 μm.The width is defined to extend from r₃ to a radius r₄ where Δ_(CORE4)(r)reaches 0.15% and r₄>r₃. Preferably, Δ_(CORE4)(r) is less than 0.1% forall radii greater than 8 μm. Preferably, the cladding surrounds and isdirectly adjacent to the ring segment 14. Preferably, the relativerefractive index is less than 0.03% for all radii between 9 and 25 μm.Preferably, the core ends and the cladding begins at r_(CORE) of greaterthan about 8 μm, more preferably between 8 and 15 μm. The core thuscomprises three segments, a central segment 10 contactingly surroundedby a moat segment 12, the moat segment being contactingly surrounded bya ring segment 14.

In preferred embodiments, optical fiber such as those illustrated byExamples 6–8 disclosed herein preferably have: a negative dispersion at1550 nm having an absolute magnitude of less than 7.5 ps/nm-km, morepreferably between 1 and 7.5 ps/nm-km, even more preferably between 1and 7 ps/nm-km; a dispersion slope at 1550 nm of about 0.13 to 0.135ps/nm²-km; dispersion zero between 1550 and 1600 nm; an opticaleffective area at 1550 nm less than 80 μm², more preferably between 50and 80 μm², even more preferably between 60 μm² and 80 μm²; an opticalmode field diameter at 1550 nm of less than 10.5 μm; and pin array bendloss at 1550 nm less than 10 dB. Preferably, attenuation at 1550 nm isless than 0.21 dB/km, more preferably less than 0.20 dB/km.

Preferably, the part of the relative refractive index that includesΔ_(4MAX) in the fourth portion of the core is relatively flat, i.e. ofrelatively constant value. Preferably, the absolute magnitude of thedifference in Δ(r) between any radii between r=6 μm and r=7 μm is lessthan 0.07%, more preferably less than 0.05%.

A_(L01) may be greater than A_(L02), or A_(L02) may be greater thanA_(L01), or A_(L02) may be substantially equal to A_(L02). Preferably,A_(L01) and A_(L02) are both greater than 150 μm², more preferablygreater than 175 μm², and even more preferably greater than 200 μm².

In preferred embodiments, 0.4<AOEA_(L01)/AOEA_(L02)<2.5. In otherpreferred embodiments, 0.5<AOEA_(L01)/AOEA_(L02)<2.

Cladding 200, or a cladding layer, shown in FIG. 3 where Δ_(CLAD)=0,surrounds and is directly adjacent the outer region of the core.

Examples 6–8 can thus be described as having a core comprising a centralsegment 10, a moat segment surrounding and directly adjacent to thecentral segment, and a ring segment 14 surrounding and directly adjacentto the moat segment. The core preferably has an entirely positiverefractive index. Preferably, the inner region comprises the maximumΔ_(CORE)(i.e. Δ_(MAX)) for the entire fiber, the intermediate regioncomprises Δ_(3MIN), and the outer region comprises Δ_(4MAX), whereΔ_(MAX)>Δ_(4MAX)>Δ_(3MIN)>0. Preferably, the relative refractive indexprofile in the first portion comprises a substantially flat part. Thethird portion comprises Δ_(4MAX) which is less than Δ_(MAX). Preferably,the relative refractive index profile in the fourth portion comprises asubstantially flat part, and more preferably, a substantial part of thefourth portion has a substantially flat relative refractive indexprofile Δ_(CORE4)(r).

Preferably, the optical fiber disclosed herein has an absolute thresholdof greater than 9.5 dBm, more preferably greater than 10.0 dBm, evenmore preferably greater than 10.5 dBm, for fiber lengths greater than orequal to about 50 km.

Preferably, the optical fiber disclosed herein has an attenuation at1380 μm which is not more than 0.3 dB/km greater than an attenuation at1310 μm, more preferably not more than 0.1 dB/km greater, even morepreferably not more than 0.05 dB/km greater. In preferred embodiments,the attenuation at 1380 nm is not more than the attenuation at 1310 nm.In other preferred embodiments, the attenuation at 1380 nm is less than0.3 dB/km. In a preferred set of embodiments, the absolute SBS thresholdis greater than 8.5+10 log[(1−e^(−(0.19)(50)/4.343))/(1−e^(−(α)(L)/4.343))] dBm, preferablygreater than 9+10 log [(1−e^(−(0.19)(50)/4.343))/(1−e^(−(α)(L)/4343))]dBm, even more preferably greater than 9.5+10 log[(1−e^(−(0.19)(50)/4.343))/(1−e^(−(α)(L)/)4.343)] dBm (where L is thelength of the fiber in km and α is the attenuation of the fiber at 1550nm) and the attenuation at 1380 μm is not more than 0.3 dB/km greaterthan an attenuation at 1310 μm, more preferably not more than 0.1 dB/kmgreater, even more preferably not more than 0.05 dB greater, and inpreferred embodiments the attenuation at 1380 nm is not more than theattenuation at 1310 nm. In other preferred embodiments, the attenuationat 1380 nm is less than 0.3 dB/km. In some preferred embodiments, theoptical effective area at 1550 nm is greater than 80 μm, and in otherpreferred embodiments, the optical effective area at 1550 nm is greaterthan 80 μm² and less than 110 μm².

The optical fiber disclosed herein preferably exhibits a PMD of lessthan 0.1 ps/sqrt(km), more preferably 0.05 ps/sqrt(km), and even morepreferably less than 0.02 ps/sqrt(km). In preferred embodiments, the pinarray bend loss at 1550 nm is less than 5 dB, more preferably less than3 dB. In preferred embodiments, the pin array bend loss at 1620 nm isless than 10 dB, more preferably less than 7 dB, more preferably lessthan 5 dB.

Preferably, the optical fiber disclosed herein has a cabled cutoff ofless than 1400 rum, more preferably between 1200 and 1300 nm.

Preferably, the optical fiber disclosed herein is capable oftransmitting optical signals in the 1260 nm to 1625 nm wavelength range.

Preferably, the fibers disclosed herein are made by a vapor depositionprocess. Even more preferably, the fibers disclosed herein are made byan outside vapor deposition (OVD) process. Thus, for example, known OVDlaydown, consolidation, and draw techniques may be advantageously usedto produce the optical waveguide fiber disclosed herein. Otherprocesses, such as modified chemical vapor deposition (MCVD) or vaporaxial deposition (VAD) or plasma chemical vapor deposition (PCVD) may beused. Thus, the refractive indices and the cross sectional profile ofthe optical waveguide fibers disclosed herein can be accomplished usingmanufacturing techniques known to those skilled in the art including,but in no way limited to, OVD, VAD and MCVD processes.

FIG. 4 is a schematic representation (not to scale) of an opticalwaveguide fiber 300 as disclosed herein having core 100 and an outerannular cladding or outer cladding layer or clad layer 200 directlyadjacent and surrounding the core 100.

Preferably, the cladding contains no germania or fluorine dopantstherein. More preferably, the cladding 200 of the optical fiberdisclosed herein is pure or substantially pure silica. The clad layer200 may be comprised of a cladding material which was deposited, forexample during a laydown process, or which was provided in the form of ajacketing, such as a tube in a rod-in-tube optical preform arrangement,or a combination of deposited material and a jacket. The clad layer 200may include one or more dopants. The clad layer 200 is surrounded by aprimary coating P and a secondary coating S. The refractive index of thecladding 200 is used to calculate the relative refractive indexpercentage as discussed elsewhere herein.

Referring to the Figures, the clad layer 200 has a refractive index ofn_(c) surrounding the core which is defined to have a Δ(r)=0%, which isused to calculate the refractive index percentage of the variousportions or regions of an optical fiber or optical fiber preform.

As shown in FIG. 5, an optical fiber 300 as disclosed herein may beimplemented in an optical fiber communication system 30. System 30includes a transmitter 34 and a receiver 36, wherein optical fiber 300allows transmission of an optical signal between transmitter 34 andreceiver 36. System 30 is preferably capable of 2-way communication, andtransmitter 34 and receiver 36 are shown for illustration only. Thesystem 30 preferably includes a link which has a section or a span ofoptical fiber as disclosed herein. The system 30 may also include one ormore optical devices optically connected to one or more sections orspans of optical fiber as disclosed herein, such as one or moreregenerators, amplifiers, or dispersion compensating modules. In atleast one preferred embodiment, an optical fiber communication systemaccording to the present invention comprises a transmitter and receiverconnected by an optical fiber without the presence of a regeneratortherebetween. In another preferred embodiment, an optical fibercommunication system according to the present invention comprises atransmitter and receiver connected by an optical fiber without thepresence of an amplifier therebetween. In yet another preferredembodiment, an optical fiber communication system according to thepresent invention comprises a transmitter and receiver connected by anoptical fiber having neither an amplifier nor a regenerator nor arepeater therebetween.

Preferably; the optical fibers disclosed herein have a low watercontent, and preferably are low water peak optical fibers, i.e. havingan attenuation curve which exhibits a relatively low, or no, water peakin a particular wavelength region, especially in the E-band.

Methods of producing low water peak optical fiber can be found in PCTApplication Publication Numbers WO00/64825, WO01/47822, and WO02/051761,the contents of each being hereby incorporated by reference.

A soot preform or soot body is preferably formed by chemically reactingat least some of the constituents of a moving fluid mixture including atleast one glass-forming precursor compound in an oxidizing medium toform a silica-based reaction product. At least a portion of thisreaction product is directed toward a substrate, to form a porous silicabody, at least a portion of which typically includes hydrogen bonded tooxygen. The soot body may be formed, for example, by depositing layersof soot onto a bait rod via an OVD process.

A substrate or bait rod or mandrel is inserted through a glass body suchas a hollow or tubular handle and mounted on a lathe. The lathe isdesigned to rotate and translate the mandrel in close proximity with asoot-generating burner. As the mandrel is rotated and translated,silica-based reaction product, known generally as soot, is directedtoward the mandrel. At least a portion of silica-based reaction productis deposited on the mandrel and on a portion of the handle to form abody thereon.

Once the desired quantity of soot has been deposited on the mandrel,soot deposition is terminated and the mandrel is removed from the sootbody.

Upon removal of the mandrel, the soot body defines a centerline holepassing axially therethrough. Preferably, the soot body is suspended bya handle on a downfeed device and positioned within a consolidationfurnace. The end of the centerline hole remote from the handle ispreferably fitted with a bottom plug prior to positioning the soot bodywithin the consolidation furnace. Preferably, the bottom plug ispositioned and held in place with respect to the soot body by frictionfit. The plug is further preferably tapered to facilitate entry and toallow at least temporary affixing, and at least loosely, within the sootbody.

The soot body is preferably chemically dried, for example, by exposingsoot body to a chlorine-containing atmosphere at elevated temperaturewithin consolidation furnace. A chlorine-containing atmosphereeffectively removes water and other impurities from soot body, whichotherwise would have an undesirable effect on the properties of theoptical waveguide fiber manufactured from the soot body. In an OVDformed soot body, the chlorine flows sufficiently through the soot toeffectively dry the entire preform, including the centerline regionsurrounding centerline hole.

Following the chemical drying step, the temperature of the furnace iselevated to a temperature sufficient to consolidate the soot blank intoa sintered glass preform, preferably about 1500° C. The centerline holeis then closed during the consolidation step so that the centerline holedoes not have an opportunity to be rewetted by a hydrogen compound priorto centerline hole closure. Preferably, the centerline region has aweighted average OH content of less than about 1 ppb.

Exposure of the centerline hole to an atmosphere containing a hydrogencompound can thus be significantly reduced or prevented by closing thecenterline hole during consolidation.

As described above and elsewhere herein, the plugs are preferably glassbodies having a water content of less than about 31 ppm by weight, suchas fused quartz plugs, and preferably less than 5 ppb by weight, such aschemically dried silica plugs. Typically, such plugs are dried in achlorine-containing atmosphere, but an atmosphere containing otherchemical drying agents are equally applicable. Ideally, the glass plugswill have a water content of less than 1 ppb by weight. In addition, theglass plugs are preferably thin walled plugs ranging in thickness fromabout 200 μm to about 2 mm. Even more preferably, at least a portion ofa top plug has a wall thickness of about 0.2 to about 0.5 mm. Morepreferably still, elongated portion 66 has a wall thickness of about 0.3mm to about 0.4 mm. Thinner walls promote diffusion, but are moresusceptible to breakage during handling.

Thus, inert gas is preferably diffused from the centerline hole afterthe centerline hole has been sealed to create a passive vacuum withinthe centerline hole, and thin walled glass plugs can facilitate rapiddiffusion of the inert gas from the centerline hole. The thinner theplug, the greater the rate of diffusion. A consolidated glass preform ispreferably heated to an elevated temperature which is sufficient tostretch the glass preform, preferably about 1950° C. to about 2100° C.,and thereby reduce the diameter of the preform to form a cylindricalglass body, such as a core cane or an optical fiber, wherein thecenterline hole collapses to form a solid centerline region. The reducedpressure maintained within the sealed centerline hole created passivelyduring consolidation is generally sufficient to facilitate completecenterline hole closure during the draw (or redraw) process.Consequently, overall lower O—H overtone optical attenuation can beachieved. For example, the water peak at 1383 nm, as well as at other OHinduced water peaks, such as at 950 nm or 1240 nm, can be lowered, andeven virtually eliminated.

A low water peak generally provides lower attenuation losses,particularly for transmission signals between about 1340 nm and about1470 nm. Furthermore, a low water peak also affords improved pumpefficiency of a pump light emitting device which is optically coupled tothe optical fiber, such as a Raman pump or Raman amplifier which mayoperate at one or more pump wavelengths. Preferably, a Raman amplifierpumps at one or more wavelengths which are about 100 nm lower than anydesired operating wavelength or wavelength region. For example, anoptical fiber carrying an operating signal at wavelength of around 1550nm may be pumped with a Raman amplifier at a pump wavelength of around1450 nm. Thus, the lower fiber attenuation in the wavelength region fromabout 1400 nm to about 1500 nmn would tend to decrease the pumpattenuation and increase the pump efficiency, e.g. gain per mW of pumppower, especially for pump wavelengths around 1400 nm. Generally, forgreater OH impurities in a fiber, the water peak grows in width as wellas in height. Therefore, a wider choice of more efficient operation,whether for operating signal wavelengths or amplification with pumpwavelengths, is afforded by the smaller water peak. Thus, reducing OHimpurities can reduce losses between, for example, for wavelengthsbetween about 1260 nm to about 1650 nm, and in particular reduced lossescan be obtained in the 1383 nm water peak region thereby resulting inmore efficient system operation.

The fibers disclosed herein exhibit low PMD values particulary whenfabricated with OVD processes. Spinning of the optical fiber may alsolower PMD values for the fiber disclosed herein.

Brillouin scattering loss of the optical fiber disclosed herein, and inparticular of Ge-doped optical fiber, may be further reduced bymodulating the tension applied to the fiber during draw. At least aportion, preferably an end portion, of an optical fiber preform isheated to a high temperature so that an optical fiber can be drawn, suchas by lowering the preform into an RF induction furnace and heating itto a melting temperature, the preform comprising a high purity, low lossgermanium silicate glass core surrounded by an outer layer of glasscladding with a lower index of refraction than the core. Fiber is thendrawn from the heated preform at an appropriately modulated tension.Upon sufficient heating, a melted end portion of the preform bearing aglass strand drops, and the strand is inserted into a fiber drawingstation. The parameters are then adjusted to produce a fiber of desireddiameter and uniformity. The fiber drawing speed and tension can beunder control of a computer. the draw tension on the fiber is modulatedwith respect to fiber length in a sinusoidal, triangular or, preferably,a trapezoidal waveform essentially between a minimum in the range 10 to50 g and a maximum in the range 150 to 250 g. The sinusiodal waveform isactually the positive half of a true sinusoid, and its wavelength asreferred to herein is the length from the minimum tension range to themaximum back to the minimum. The preferred wavelength of a sinusoidal isin the range 3 to 30 km. The preferred triangular waveform ischaracterized by a base along the length in the range 3 to 30 km; andthe preferred trapezoidal waveform has a pair of bases along the fiberlength: a major base in the range 3 km to 15 km and a minor base in therange 1 km to 13 km. The resulting product is drawn optical fiber havinga Ge-doped core and a cladding surrounding the core. The core ischaracterized by a repeated pattern of modulated strain. The strain ismodulated with length between a low produced by 10–50 g of stress in thedraw to a high produced by 150–250 g stress in the draw. The modulationpattern is characterized by a repetition length in the range 3 to 30km,. The pattern waveform is preferably sinusoidal, triangular ortrapezoidal. Also see U.S. Pat. No. 5,851,259, which is incorporatedherein by reference in its entirety.

All of the optical fibers disclosed herein can be employed in an opticalsignal transmission system, which preferably comprises a transmitter, areceiver, and an optical transmission line. The optical transmissionline is optically coupled to the transmitter and receiver. The opticaltransmission line preferably comprises at least one optical fiber span,which preferably comprises at least one section of optical fiber.

The system preferably further comprises at least one amplifier, such asa Raman amplifier, optically coupled to the optical fiber section.

The system further preferably comprises a multiplexer forinterconnecting a plurality of channels capable of carrying opticalsignals onto the optical transmission line, wherein at least one, morepreferably at least three, and most preferably at least ten opticalsignals propagate at a wavelength between about 1260 nm and 1625 nm.Preferably, at least one signal propagates in one or more of thefollowing wavelength regions: the 1310 nm band, the E-band, the S-band,the C-band, and the L-band.

In some preferred embodiments, the system is capable of operating in acoarse wavelength division multiplex mode wherein one or more signalspropagate in at least one, more preferably at least two of the followingwavelength regions: the 1310 nm band, the E-band, the S-band, theC-band, and the L-band.

In one preferred embodiment, the system comprises a section of opticalfiber as disclosed herein having a length of not more than 20 km. Inanother preferred embodiment, the system comprises a section of opticalfiber as disclosed herein having a length of greater than 20 km. In yetanother preferred embodiment, the system comprises a section of opticalfiber as disclosed herein having a length of greater than 70 km.

In one preferred embodiment, the system operates at less than or equalto about 1 Gbit/s. In another preferred embodiment, the system operatesat less than or equal to about 2 Gbit/s. In yet another preferredembodiment, the system operates at less than or equal to about 10Gbit/s. In still another preferred embodiment, the system operates atless than or equal to about 40 Gbit/s. In yet another preferredembodiment, the system operates at greater than or equal to about 40Gbit/s.

In a preferred embodiment, a system disclosed herein comprises anoptical source, an optical fiber as disclosed herein optically coupledto the optical source, and a receiver optically coupled to the opticalfiber for receiving the optical signals transmitted through the opticalfiber, the optical source having the capability of dithering, and/orphase modulating, and/or amplitude modulating, the optical signalgenerated by the optical source, and the optical signal is received bythe receiver.

Stimulated Brillouin scattering (SBS) can be measured by a measurementsystem that records input power (P_(in)) and backscattered power(P_(bs)) as input power is varied over a defined range of input powers.Various systems and/or methods of determining the SBS threshold of anoptical fiber could be used to characterize the fiber. One preferredmethod and system are disclosed herein.

The measurement system disclosed herein comprises a light source, anerbium-doped fiber amplifier (EDFA), a variable optical attenuator(VOA), a polarization controller, an optical power routing device suchas a two-by-two coupler or an optical circulator, and several opticalpower detectors and power meters. Single-mode patchcords with FC/APCconnectors join these components. A representative measurement system isshown in FIG. 6.

The light source, which may be a tunable or single-wavelength continuouswave laser, has a very narrow spectral width, about 150 kHz or less. Thewavelength is preferably centered around 1550 nm, but can vary withinthe gain band of the EDFA. An EDFA is used to amplify the optical signalto power levels that can induce SBS in the fiber under test. A variableoptical attenuator (VOA) is used to vary the optical power that islaunched into the fiber under test. The VOA is selected to allowsufficiently fine step sizes and sufficient range to allow themeasurement of input power and backscattered power across a broad rangeof input powers. A polarization control device is preferably used toestablish 100% degree of polarization and a stable state ofpolarization. A two-by-two directional coupler or optical circulatordirects power to the fiber under test and supports the monitoring ofbackscattered power (Port B) and/or input power (Port A). The fiberunder test (FUT) is connected to the coupler or circulator with a fusionsplice or other reflectionless connection device or method. A thirddetector may be used to monitor output power at Port C. Unless otherwisenoted herein, SBS threshold values reported herein correspond tosubjecting the optical fiber to the output of a continuous wave laserhaving a very narrow spectral width, about 150 kHz or less. Higherthreshold values may be obtained for the same fiber when subjected tothe output of sources having dithered or wider spectral widths. SBSthreshold values reported herein correspond to optical fibers having alength of about 50 km, unless otherwise noted. It should be understoodthat the SBS threshold measurements could be performed on differentlengths of fiber.

To conduct a measurement, a fiber is spliced into the system and thecoupler taps are connected to the optical power detectors. The laser isactivated and the EDFA yields a fixed output power. The VOA attenuationis stepped across a selected range in small increments, from a highinserted loss value to zero. For example, in one embodiment the stepsize is 0.1 dB and the scan range is 20 dB.

Reference measurement is conducted to obtain the actual input power.Although the input power is monitored during this process, the referencemeasurement allows a determination of actual input power without havingto account for polarization dependent loss (PDL) and splice loss. Thismeasurement is conducted on a two-meter sample of the fiber under test.The fiber is cutback and connected to Port C. The VOA scan is repeatedover the same range, and the reference input power is recorded at PortC. These power values are used as the input powers of record. The inputpower and backscattered power level are recorded at each step (see curveP in FIG. 7).

When the scans are completed, first and second derivatives of the curveare calculated. The data set are preferably smoothed prior tocalculating the first and second derivatives. The absolute SBS thresholdis herein defined at the point at which the second derivative ismaximum. An illustrative plot of measured data (curve P) and the firstand second derivatives (curve P′ and P″, respectively) are presented inFIG. 7. Curve P′ is then the first derivative of backscattered power inmW with respect to input power in mW. Curve P″ is the second derivativeof backscattered power in mW with respect to input power in mW. In FIG.7, the abscissa of the peak P″_(PEAK) of curve P″ is the absolute SBSthreshold, SBSt, in dBm (e.g. 8.22 dBm in FIG. 7). That is, the inputpower at which the second derivative is a maximum is defined as theabsolute SBS threshold for the fiber.

As reported herein, SBS threshold values were obtained with apolarization control device that establishes a fixed polarization state.However, in an alternate embodiment of the system and/or method formeasuring SBS threshold, the SBS threshold could also be measured with apolarization randomizer or scrambler. The use of a polarizationrandomizer would increase the measured SBSt values for a given opticalfiber by approximately a factor of 1.5 [see M. O. van Deventer and A. J.Boot, J. Lightwave Technology, vol. 12, pp. 585–590 (1994)] whencompared to the SBSt value obtained with a fixed polarization state(100% degree of polarization and constant state of polarization).

Comparative SBS threshold values reported herein, such as SBS thresholdimprovement over a representative SMF-28™ optical fiber manufactured byCorning Incorporated which has an attenuation similar to the attenuationof the optical fibers disclosed herein, compare the SBS threshold ofdifferent fibers of the same length measured in the same way (i.e. bythe same method, and measurement system if measurement data is used).Thus, even though various SBS threshold measurement methods (andsystems) may exist, the comparative values obtained from two differentfibers according to the same method should be substantially similar tocomparative values obtained from those fibers utilizing a differentmethod.

The SBS threshold varies with the length and attenuation of the fiberunder test. Generally, a very short length of an optical fiber will tendto have a higher SBS threshold value than a very long length of the samefiber. Also, generally, a length of one optical fiber having a higherattenuation will tend to have a higher SBS threshold value than the samelength of another similar optical fiber having a lower attenuation. Anapproximate analytical expression is given in “Raman and BrillouinNon-Linearities in Broadband WDM-Overlay Single Fiber PONs,” G. H.BuAbbud et al., ECOC 2003:

${{P_{th}(L)} \approx {21\frac{\alpha\; A_{eff}}{g_{B}^{eff}\left\lbrack {1 - {\exp\left( {{- \alpha}\; L} \right)}} \right\rbrack}}},$where g_(B) ^(eff) is the effective Brillouin gain coefficient, α is theattenuation, L is the fiber length, A_(eff) is the optical effectivearea. In this simple approximation, the SBS threshold is inverselyproportion to the effective length of the fiber. Thus, if the measuredthreshold for a length L₁ is P₁, then the threshold at length L₂ is

${P_{2}({dB})} \cong {{P_{1}({dB})} + {10\mspace{20mu}{{\log\left\lbrack \frac{1 - {\exp\left( {{- \alpha}\; L_{1}} \right)}}{1 - {\exp\left( {{- \alpha}\; L_{2}} \right)}} \right\rbrack}.}}}$

For example, the values of SBS threshold reported herein correspond tofibers having a length (L₁) of about 50 km and an attenuation at 1550 nmof about 0.19 dB/km. Thus, the SBS threshold P₂ for an optical fiber ofthe type disclosed herein having a length L₂ and attenuation α₂ can bedetermined from:

${P_{2}({dB})} \cong {{P_{1}({dB})} + {10\mspace{11mu}{{\log\mspace{11mu}\left\lbrack \frac{1 - {\exp\left( {- \left( {0.19*{50.5/4.343}} \right)} \right)}}{1 - {\exp\left( {{- \alpha}\; L_{2}} \right)}} \right\rbrack}.}}}$

Preferably, the optical fiber disclosed herein has a silica-based coreand cladding. In preferred embodiments, the cladding has an outerdiameter of about 125 μm. Preferably, the outer diameter of the claddinghas a constant diameter along the length of the optical fiber. Inpreferred embodiments, the refractive index of the optical fiber hasradial symmetry.

It is to be understood that the foregoing description is exemplary ofthe invention only and is intended to provide an overview for theunderstanding of the nature and character of the invention as it isdefined by the claims. The accompanying drawings are included to providea further understanding of the invention and are incorporated andconstitute part of this specification. The drawings illustrate variousfeatures and embodiments of the invention which, together with theirdescription, serve to explain the principals and operation of theinvention. It will become apparent to those skilled in the art thatvarious modifications to the preferred embodiment of the invention asdescribed herein can be made without departing from the spirit or scopeof the invention as defined by the appended claims.

1. An optical fiber comprising: a length; a core having a refractiveindex profile and a centerline; the core refractive index comprising acentral segment, a moat segment surrounding the central segment, and aring segment surrounding the moat segment, the central segment having agreater relative refractive index than the ring segment, the ringsegment having a greater relative refractive index than the moatsegment, and a cladding layer surrounding and directly adjacent thecore; wherein the optical fiber has an attenuation at 1550 nm; whereinthe refractive index of the core is selected to provide: an opticaleffective area at 1550 nm less than 80 μm²; a dispersion at 1550 nmhaving an absolute magnitude less than 7.5 ps/nm-km; and an absolute SBSthreshold in dBm greater than about 9.5+10 log[(1−e^(−(0.19)(50)/4.343))/(1−e^(−(α)(L)/4.343))], wherein L is thelength in km and α is the attenuation in dB/km at 1550 nm.
 2. Theoptical fiber of claim 1 wherein the optical effective area is between50 and 80 μm².
 3. The optical fiber of claim 1 wherein the opticaleffective area is between 60 and 80 μm².
 4. The optical fiber of claim 1wherein the absolute SBS threshold in dBm is greater than about 10+10log [(1−e^(−(0.19)(50)/4.343))/(1−e^(−(α)(L)/4.343))].
 5. The opticalfiber of claim 1 wherein the absolute SBS threshold in dBm is greaterthan about 10.5+10 log[(1−e^(−(0.19)(50)/4.343))/(1−e^(−(α)(L)/4.343))].
 6. The optical fiberof claim 1 wherein the dispersion at 1550 nm is positive.
 7. The opticalfiber of claim 1 wherein the dispersion at 1550 nm is negative.
 8. Theoptical fiber of claim 1 wherein the acousto-optic effective area,AOEA_(L01), of the L₀₁ acoustical mode is not less than 150 μm at theBrillouin frequency of the optical fiber.
 9. The optical fiber of claim1 wherein the acousto-optic effective area, AOEA_(L02), of the L₀₂acoustical mode is not less than 150 μm² at the Brillouin frequency ofthe optical fiber.
 10. The optical fiber of claim 1 wherein the moatsegment is adjacent to and contacts the central segment, and a ringsegment is adjacent to and contacts the moat segment.
 11. The opticalfiber of claim 1 wherein the core comprising a maximum relativerefractive index greater than 0.75%.
 12. An optical transmission systemcomprising a transmitter, a receiver, and the optical fiber of claim 1optically coupled at one end to the transmitter and optically coupledanother end to the receiver.
 13. An optical fiber comprising: a length;a core having a refractive index profile and a centerline; and acladding layer surrounding and directly adjacent the core; wherein theoptical fiber has an attenuation at 1550 nm; wherein the refractiveindex of the core is selected to provide: an optical effective area at1550 nm less than 80 μm²; a negative dispersion at 1550 nm; and anabsolute SBS threshold in dBm greater than about 9.5+10 log[(1−e^(−(0.19)(50)/4.343))/(1−e^(−(α)(L)/4.343))], wherein L is thelength in km and α is the attenuation in dB/km at 1550 nm.
 14. Anoptical transmission system comprising a transmitter, a receiver, andthe optical fiber of claim 13 optically coupled at one end to thetransmitter and optically coupled another end to the receiver.
 15. Anoptical fiber comprising: a length; a core having a refractive indexprofile and a centerline; the core refractive index comprising a firstportion having relative refractive index Δ₁, a second portion havingrelative refractive index Δ₂ and a third portion having relativerefractive index Δ₃, wherein Δ₁>Δ₃>Δ₂ a cladding layer surrounding anddirectly adjacent the core; wherein the optical fiber has an attenuationat 1550 nm; wherein the refractive index of the core is selected toprovide: an optical effective area at 1550 nm less than 80 μm²; adispersion at 1550 nm having an absolute magnitude less than 7.5ps/nm-km; and an absolute SBS threshold in dBm greater than about 9.5+10log [(1−e^(−(0.19)(50)/4.343))/(1−e^(−(α)(L)/4.343))], wherein L is thelength in km and α is the attenuation in dB/km at 1550 nm.